Pakistan has suspended mobile calls and data services as millions head to the polls to vote in a new government.
An interior ministry spokesman said the measure was warranted, citing recent incidents of terror in the country.
The election comes almost two years since the previous prime minister, cricketer-turned-politician Imran Khan, was ousted in a no-confidence vote.
Three-time PM Nawaz Sharif is now on the ballot in what many analysts say is Pakistan’s least credible election yet.
Khan was jailed on corruption charges last year and is barred from standing.
Both calls and data services have been suspended, though wifi networks still appear to be working.
One voter told the BBC they were shocked at the decision, saying “voters should be facilitated instead of [having to be met with] such hurdles”.
Another said she was expecting a blanket shutdown.
Many voters in the city of Lahore told the BBC that the internet blackout meant it was not possible to book taxis to go and vote, while others said they couldn’t chat to other family members to co-ordinate when to head to polling stations.
Justifying the move, an Interior Ministry spokesman said: “As a result of the recent incidents of terrorism in the country, precious lives have been lost. Security measures are essential to maintain law and order situation and to deal with potential threats.”
Two bomb blasts killed 28 people in Balochistan province on Wednesday.
The shutdown was also criticised by Bilawal Bhutto Zadari, son of murdered ex-Prime Minister Benazir Bhutto, who called for services to be restored “immediately”. Mr Bhutto, who is also running for the top job, said his Pakistan People’s Party (PPP) had approached the election commission and the courts to get services restored.
The country is on high alert, with a heavy security presence at polling stations across the country. One station in Lahore the BBC visited had armed guards at the entrance and army officers roaming around the area.
Border crossings with Afghanistan and Iran have been closed for both cargo and pedestrians to “ensure full security” during polling, a spokesman from Pakistan’s foreign ministry said.
The country has in the past cut internet services to control the flow of information – though a shutdown of this extent is unprecedented, especially during an election.
Absolutely fair election: Nawaz Sharif
Mr Sharif and his daughter Maryam voted in Icchra, Lahore on Thursday afternoon. Security was tight, with officers forming a ring around them and a jeep covered in antennas to jam phone signals.
Black cars lined the area as the pair entered the station.
When asked if he thought the election was free and fair, Mr Sharif said they were “absolutely fair”.
Speaking to the BBC outside the polling station after having cast his vote, he said he had “never had any problems with the military”.
Mr Sharif spoke of the “lack of civility, the arrogance, and this culture of disrupting and destroying the country”, in an apparent reference to Pakistan under Mr Khan.
He said he and his family had gone to jail, “made sacrifices and now we are here witnessing this day”. If his party wins, “people’s lives will become easier, inflation will go down – this is what people want, this is their wish – and their wishes should come true”, he added.
Voting ends at 1700 local time (1200 GMT). Strict rules around election coverage – including what can be said about candidates, campaigning and opinion polls – remain in place until 23:59 local time on Thursday. Its unclear how soon results will be announced but they must be released within two weeks of the vote.
Outside one polling station in the city of Multan in Punjab, some female polling agents told the BBC they were not allowed to enter polling booths – and therefore could not observe the polling process.
Typically, female polling agents are given a seat inside booths.
In Lahore, dozens of voters crammed into the small corridors of a school in Naseerabad, with some saying they had been waiting for more than two hours to vote.
Rising violence and economic struggles
As many as 128 million people are registered to cast their votes, almost half of whom are under the age of 35. More than 5,000 candidates – of whom just 313 are women – are contesting the 266 directly-elected seats in the 336-member National Assembly.
The Pakistan Muslim League (Nawaz) (PML-N) and the PPP are considered the two major parties going into the vote.
However, picking out candidates from Khan’s Pakistan Tehreek-e-Insaf (PTI) party is more difficult, after it was banned from using the cricket bat symbol under which all its candidates run.
The move has forced PTI-backed candidates, who are running as independents, to use other symbols instead, including calculators, electric heaters and dice. Electoral symbols play a key role in a country where more than 40% are unable to read.